The purpose of this investigation is: i. To test the hypothesis that tremors and othr abnormal involuntary movements may be attenuated with statistical and clinical significance by application of appropriate mechanical loads and to determine for particular disorders the optimal loading functions; ii. To use the motor performance testing apparatus to establish a correlation, if one exists, between the effectiveness of various clinical therapies for tremors and the response of these tremors to mechanical loading, with a view toward clinical predictive tremor assessment; iii. To interpret the variation of amplitude and spectral characteristics of tremor with applied load to test the validity of models for tremor mechanism, i.e., central oscillator, mechanical resonance and reflex instability, as applied to pathological tremors. Loading-simulating torques will be applied by torque motors under closed-loop digital control through a "loading joystick" during two-dimensional tracking tasks. Load parameters will be adjusted automatically by an on-line search algorithm so as to determine the values which minimize tremor with respect to purposeful movement.